A great deal of interest is focused on a freezing cycle that uses carbon dioxide (CO.sub.2) as one of the non-freon freezing cycles proposed as alternatives to a freezing cycle (freon cycle) that utilizes freon. While a freon cycle in the prior art requires a liquid reservoir such as a liquid tank to be provided in the high-pressure line in order to absorb fluctuations of the load and leaking of the coolant gas occurring over time, a CO.sub.2 cycle, in which the temperature on the high-pressure side exceeds the critical point (31.1.degree. C.), unlike in the freon cycle, does not allow a liquid tank to be provided in the high-pressure line, thus necessitating an accumulator to be provided on the downstream side relative to the evaporator.
As a result, since the liquid reservoir is provided on the downstream side relative to the evaporator, superheat control such as that adopted in a freon cycle cannot be implemented, and instead, a system that is capable of controlling the high-pressure must be provided.
In addition, since the freezing capability and the COP (coefficient of performance: freezing effect/compressor work) of a CO.sub.2 cycle are inferior to those achieved by a freon cycle, the cycle structure as illustrated in Japanese Examined Patent Publication No. H 7-18602 may be adopted to improve the freezing capability and the COP.
To explain this cycle structure in reference to FIG. 5, a freezing cycle 1 that utilizes CO.sub.2 is provided with a compressor 2 that raises the pressure of a coolant, a radiator 3 that cools down the coolant, an internal heat exchanger 4 that performs heat exchange for coolant flowing through a high-pressure line and a low-pressure line, an expansion valve 5 that reduces the pressure of the coolant, an evaporator 6 that evaporates and gasifies the coolant and an accumulator 7 that achieves gas/liquid separation for the coolant flowing out of the evaporator. In this cycle, the coolant in a supercritical state with its pressure having been raised at the compressor 2 is cooled down by the radiator 3 and is further cooled by the internal heat exchanger 4 before it enters the expansion valve 5. The pressure of the coolant thus cooled is reduced at the expansion valve 5 and thus the coolant becomes moist steam. After the coolant is evaporated at the evaporator 6, gas/liquid separation is achieved by the accumulator 7, and then heat exchange with the high-pressure side coolant is performed by the internal heat exchanger 4 so that the coolant becomes heated before it is returned to the compressor 2.
These changes in the state of the cycle are as indicated as A.fwdarw.B.fwdarw.C.fwdarw.D.fwdarw.E .fwdarw.F.fwdarw.A in the Mollier diagram in FIG. 6, with the coolant indicated by point A becoming compressed at the compressor 2 to become high-temperature, high-pressure coolant in the supercritical state indicated by point B, the high-temperature, high-pressure coolant cooled down to point C by the radiator 3 and further cooled down to point D by the internal heat exchanger 4. Then its pressure is reduced at the expansion valve 5 and the coolant becomes moist steam at a low temperature and a low pressure, as indicated by point E. Next, it becomes evaporated and gasified at the evaporator 6 before reaching point F. The coolant having passed through the evaporator 6 is further heated by the internal heat exchanger 4 up to point A, and then is compressed again by the compressor 2.
Thus, the cycle provided with the internal heat exchanger 4 achieves a freezing effect which is greater by the enthalpy difference between point E and point E' compared to the freezing effect achieved by a cycle without the internal heat exchanger 4 (F-B'-C-E'-F), and since the work performed by the compressor (the enthalpy difference between point A and point G) does not fluctuate greatly whether or not the internal heat exchanger 4 is provided, the COP can be increased by providing the internal heat exchanger 4.
It is known that the freezing capability and the COP of a CO.sub.2 cycle are affected by high-pressure and that the COP is at its best at a certain pressure level (10.about.15 MPa). For instance, in the summer when the temperature of the coolant at the outlet of the gas cooler reaches approximately 40.degree. C., there is a high-pressure .beta. which allows the COP to reach a maximum value a as shown in FIG. 7.
In addition, while the presence of the internal heat exchanger 4 contributes to improving the COP as described above, it is known that there is an optimal value for the heat exchange quantity that allows the COP to reach its maximum value as shown in FIG. 8.
Accordingly, an object of the present invention is to provide a freezing cycle utilizing a supercritical fluid as a coolant and provided with an internal heat exchanger to perform heat exchange on the coolant at the outlet side of a gas cooler and at the intake side of a compressor, which is capable of achieving good cycle efficiency by maintaining an optimal high-pressure through cycle balance control. Another object of the present invention is to provide a freezing cycle which can be temporarily protected against excessively high-pressure or excessively high discharge temperature at the compressor.